The present invention relates to a process for the expansion of tobacco, wherein (a) the tobacco is cooled to a predetermined temperature by mixing it with cold carbon dioxide in a predetermined weight ratio, (b) the cooled tobacco is impregnated with liquid carbon dioxide formed by condensing gaseous carbon dioxide at a predetermined pressure and for a predetermined contact period, (c) the liquid carbon dioxide condensed in the tobacco is converted to solid carbon dioxide through rapid reduction of the carbon dioxide pressure to form simultaneously gaseous carbon dioxide, and (d) the tobacco containing solid carbon dioxide is subjected to a hot gas treatment to achieve the expansion.
Such a process is known from DE-A 34 45 752 (=U.S. Pat. No. 4,528,994). In all steps of this process, carbon dioxide is used which is present in solid, liquid or gaseous form depending on the requirements and reaction conditions. The first step is carried out with sufficient solid carbon dioxide so that in the subsequent step wherein the tobacco is treated with pressurized gaseous carbon dioxide there is present a mixture of solid carbon dioxide and tobacco. The weight ratio between solid carbon dioxide and tobacco is preferably 96%/130%, and especially 125%/128%, so that there is a sufficiently high-percentage increase of the filling capacity of the tobacco in the final expansion carried out by hot gas treatment.
Due to the very high demand for carbon dioxide, especially solid carbon dioxide, this known process is not only uneconomical but, because of the high amount of carbon dioxide used, has the disadvantage that this excess solid carbon dioxide leads, during the subsequent treatment of the mixture of solid carbon dioxide and tobacco with gaseous carbon dioxide, to the formation of excess liquid carbon dioxide. The consequence thereof is that the liquid carbon dioxide is not selectively absorbed substantially only in the pores and on the porous surface of the tobacco. Furthermore, there is involved the risk of the formation of tobacco lumps due to icing caused by excessive carbon dioxide.
A corresponding process for the expansion of tobacco is also known from DE-A 34 45 753 (=U.S. Pat. No. 4,630,619). This process differs from the process of DE-A 34 45 752 mainly by the feature that in the first step the tobacco is cooled through a treatment with liquid nitrogen, so that in the subsequent step of treatment with gaseous carbon dioxide one will obviously have to apply a relatively high pressure. In this prior art process, nitrogen is entrained during the cooling step so that in the subsequent steps there are always formed gas mixtures consisting of nitrogen and carbon dioxide. An economic recovery and recycling of carbon dioxide is, therefore, not possible. In addition, this process involves the aforementioned disadvantages of the process of DE-A 34 45 752, as again there is no selective absorption of liquid carbon dioxide. There is also involved the formation of excessive solid carbon dioxide and subsequent production of excessive liquid carbon dioxide.